It is well-known that data may be encrypted before it is either transmitted to a user or stored in memory to prevent unauthorized access to the unencrypted data. For example, cable-TV systems and direct broadcast satellite video systems typically encrypt video signals before the signals are transmitted to a user. A so-called set-top box associated with the user decrypts the signals as they are received from the service provider and supplies the decrypted signals to an associated video monitor for display when the monitor is tuned to the channel carrying the video signals. The capability to decrypt such signals is typically embodied in a decryption module disposed in the set-top box, and may be, for example, an integrated circuit or a so-called smart card which "plug" into the set-top box. The decryption key that is used to decrypt video signals characterizing a particular video program is typically supplied by the provider of the video signals in a well-known manner, e.g., via satellite or telephone line.
It is also well-known that a person may use any one of a number of different illicit means to obtain such a key to view a program that has been encrypted by the program provider. Such means include so-called video "pirates" who design and market smart cards that are able to illicitly produce a service provider's decryption key, thereby allowing a person to fraudulently access an encrypted video program. Present estimates indicate that such fraudulent access costs video program providers more than six billion dollars a year in lost revenue, and such losses are expected to increase as more programming is provided via satellite and cable TV network.